Project Summary Diffuse midline glioma (DMG), including diffuse intrinsic pontine glioma (DIPG), is a rare form of pediatric tumor that arises in the brainstem of children, and is the leading cause of death among children with a brain tumor. The current standard of care is radiotherapy, which only modestly improves survival. The lack of clinically relevant models is a major barrier to developing effective therapies for children with DMG. A novel heterozygous Lys27Met (K27M) mutation has been shown to occur in histones H3.3/3.1 in ~80% of DIPGs. However, the mechanisms by which the histone mutations promote brainstem gliomagenesis have not been fully elucidated. Several lines of evidence implicate Olig2-expressing (oligodendrocyte) progenitor cells (OPCs) in the brainstem as the likely cell-of-origin for DIPG, especially for the subtype bearing the H3.3K27M mutation. Because downstream consequences of oncogenic mutations are dependent on the tumor cell-of-origin, it is critical to develop and characterize a mouse model of midline gliomas initiated in OPCs to accelerate DIPG research and therapeutic development. Here, we propose to investigate the role of the H3.3K27M mutation in a novel DIPG mouse model where midline tumors are initiated in OPCs. Specifically, we will perform thorough tumor biologic and molecular characterization of brainstem tumors originating from OPCs, and perform a comparative analysis of Nestin cell-of-origin tumors. We hypothesize that the phenotypic and molecular effects of H3.3K27M in OPC- initiated tumors will be distinct from Nestin-initiated tumors, thus identifying novel therapeutically actionable endpoints of the histone mutation. Thus, our study will credential the first immunocompetent mouse model of DIPG initiated in OPCs, providing a novel platform to the DIPG research community to perform mechanistic and translational research. Elucidation of the downstream consequences of the histone mutation in OPCs, along with improved understanding of its cell-of-origin based effects, will help lay the foundation for future preclinical and clinical studies of DIPG and other midline gliomas based on the molecular underpinnings of the disease.